Chromatic dispersion (CD) is a deterministic distortion given by the design of the optical fiber. It leads to a frequency dependence of the optical phase and its effect on transmitted signal scales quadratically with the bandwidth consumption or equivalently the data rate. Therefore, CD tolerances are reduced to 1/16, if the data rate of a signal is increased by a factor of four (4). Up to a 2.5 Gb/s data rate optical data transmission is feasible without any compensation of CD even at long haul distances. At 10 Gb/s, the consideration of chromatic dispersion becomes necessary, and dispersion compensating fibers (DCF) are often used. At 40 Gb/s and beyond, even after the application of DCF the residual CD may still be too large for feasible optical communication.
Another transmission impairment experienced in optical transmission, for example in coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems, is Polarization-Mode Dispersion (PMD), which is a stochastic characteristic of optical fiber due to imperfections in production and installation. Pre-1990 fibers exhibit high PMD values well above 0.1 ps/√km which are border line even for 10 Gb/s. Newer fibers have a PMD lower than 0.1 ps/√km, but other optical components in a fiber link such as reconfigurable add/drop multiplexers (ROADMs) may cause substantial PMD. If 40 Gb/s systems are to be operated over the older fiber links or over new fiber links with many ROADMs, PMD may become a significant detriment.
PMD can be compensated by optical elements with an inverse transmission characteristics to the fiber. However, due to the statistical nature of PMD with fast variation speeds up to the few kHz range, the realization of optical PMD compensators is challenging. With increases in channel data rate, optical signal is more and more limited by the transmission impairments in optical fiber, such as by CD and PMD.
Thus, digital coherent detection is considered as a promising technique for future high-speed optical transmission because of its high receiver sensitivity and capability to compensate for transmission impairments such as chromatic dispersion (CD) and polarization-mode dispersion (PMD), which critically impact the performance of high-speed transmission. While 100-Gb/s Ethernet is currently being researched and developed for next-generation optical transport systems, Terabit/s Ethernet has already been mentioned as a future direction for optical transport systems.